Spherical aberration (SA) and other higher order aberrations (HOA) can be beneficial to a person's vision because it can result in an increase in depth of field. SA is a naturally occurring aberration in all human eyes, and as a result, the visual cortex has developed the ability to process a retinal image that has been slightly degraded by the presence of spherical aberration to sharpen the perceived image. Depth of field is increased by the ability of the visual cortex to process spherical aberration, and allow objects to be perceived to be in focus over a greater range of focal distances. Both negative and positive spherical aberration have a similar effect of increasing depth of field.
While some SA is beneficial, too much spherical aberration can be detrimental to the quality of vision. For example, patients with too much spherical aberration can experience decreased contrast sensitivity and night vision disturbances. The inventors of the present application have concluded that there is a maximum tolerable level of SA above which the brain's natural neural processing can no longer sharpen the retinal image to produce a clear perceived image in the mind.
Corneal excimer laser ablation profiles induce spherical aberration because of healing and biomechanical responses such as, e.g., paracentral stromal thickening and epithelial thinning, and to a lesser extent uncertainties in the ablation, e.g., due to fluence and projection. The amount of induced spherical aberration caused by the laser ablation profile is correlated to the magnitude of refraction treated, where higher refractions induce more spherical aberration. The correction of myopic refraction induces negative spherical aberration, whereas the correction of hyperopic refraction induces positive spherical aberration (using Malacara notation).
The effects of inducing spherical aberration and other higher order aberrations can also be important in patients that have been implanted with an intraocular lens (IOL). An IOL is an implanted lens in the eye to typically replace the existing crystalline lens because it has been clouded over by a cataract. Alternatively, an IOL may be implanted as a form of refractive surgery to change the optical power of the eye and correct for refractive errors. The IOL usually consists of a small synthetic lens with side struts, which are commonly referred to as haptics and are useful in holding the lens in place within the capsular bag or posterior chamber or anterior chamber of the eye. Modern IOL are made of flexible materials and are typically configured as fixed monofocal lenses designed to correct distance vision to plano although other types are available, such as multifocal IOLs and accommodative IOLs. Even though IOLs enable many patients to reduce their dependence on glasses, most patients still rely on glasses for certain activities, such as reading.
U.S. Patent Application Publication No. 2006/0195074 A1, to Bartoli describes an excimer LASER unit and relative control method for performing corneal ablation for partially correcting presbyopia that includes introducing an amount of SA to the eye of a subject. Although this and other similar methods are generally supportive to patient's vision, the uncontrolled induction of SA, for example, as described by Bartoli, has been known to result in poor contrast sensitivity for some patients, un-tolerated loss of visual acuity for far vision, or non-beneficial blur at different viewing distances.